The Second Crusade was launched from Europe and was the second major crusade which began in response to the decline of the County of Edessa. Baldwin of Boulogne found the county during the first crusade in 1098.

Pope Eugene III declared the second crusade and was the first of the crusades to be directed by European kings, Conrad III of Germany such as Louis VII of France. The armies marched separately across Europe and were each overthrown by the Seljuk Turks. The Byzantine emperor Manuel I Comnenus covertly hindered the crusader's advancement, especially in Anatolia where he is alleged to have given orders for Turks to attack them. Conrad and Louis and the fragments of their armies reached Jerusalem in 1148 and took part in an foolish attack on Damascus. The crusade in the east resulted in a failure for the crusaders and a great achievement for the Muslims.

Subsequently it would have a key impact on the decline of Jerusalem and the Third Crusade. The Second crusade was a combining force of Flemish, Norman, Scottish, Frisian and German crusaders. The army travelled from England by ship to the Holy Land and stopped helping a smaller Portuguese army in the acquisition of Lisbon. The crusader's agreed in aiding the King attack Lisbon with a serious agreement that offered them the plunder of the city's goods and the ransom money for anticipated prisoners. The Siege of Lisbon was to last from the 1 July to 25 October 1147 when the Moorish rulers agreed to surrender after four months mainly due to hunger within the city.

Later on the crusaders helped to overpower Sintra, Palmela, Almada and Setubal, where they were allowed to stay and even raise their offspring. The French crusaders met at Etampes to discuss their plans for the land route on 16 February 1147. The Germans decided upon travelling through Hungary by land as the sea route was politically impossible because of King of Sicily, Roger II was an enemy of Conrad. Many of the French nobles were wary of the land route which was through the Byzantine Empire. It was decided on the 15 of June to follow Conrad. Roger II was insulted and decided to no longer participate.

The Christian forces felt betrayed by each other and a new plan was arranged to attack Ascalon and Conrad took his army there with no further help arriving due to the lack of trust that resulted from the unsuccessful siege. This common distrust would endure for a generation due to the defeat, to the devastation of the Christian kingdoms in the Holy Land. Conrad returned to Constantinople to extend his alliance with Manuel.

Bernard of Clairvaux was embarrassed by the defeat. Bernard then sent an apology to the Pope and it is found in the second part of his Book of Consideration. The Second Crusade had long standing consequences for Jerusalem, with Damascus no longer trusting the crusader kingdom, and the city was passed onto Nur ad-Din in 1154.